Chp7

• How are elements in the periodic table arranged

  In increasing atomic (proton) number
• define periodicity

  a repeating trend in properties of elements across each period
• The number of the period gives the number of the highest energy electron shell in an elements atoms
• • elements in each group have the same n.o electrons in their outer shell
  • Elements In each group also have the same n.o electrons in each sub-shell

  therefore similarity in electron configuration gives elements in the same group similar chemistry
• Explain the periodic trend in electron configuration across period 2 and 3

  Across n=2 and n=3 the 2s sub-shell fills first with two electrons followed by the 2p sub-shell with six electrons
• For each period the s- and p-sub shells are filled in the same way. A periodic pattern
• Elements of the periodic table can also be divided into blocks corresponding to the highest energy sub-shell filled
• What group did Mendeleev completely omit

  Group 18 (0) nobel gases due to them being very unreactive